Allow `if` statement check to be configurable

[rossmacarthur]

I think it's a good idea to have this be a configurable setting, whichever the default may be. Off the top of my head, there could be a BooleanEval enum with Strict, Truthy, and Custom(Box<dyn Fn(expr, span) -> Result<bool, Error>>) that the user can set on the Engine? Or should this be another template-specific setting (#15)?

Originally posted by @lunabunn in #17 (comment)